1. Field of the Invention
The present invention relates to a sheet holding apparatus for holding in a tray a sheet (e.g., a paper, an OHP sheet, and so on) whereon an image is generated by an image forming apparatus such as a printer, a copy machine, a facsimile machine, or a multifunction apparatus having functions of at least two of the above apparatus; and an image forming system comprising the sheet holding apparatus and the image forming apparatus.
2. Description of Related Art
In general, a single or an array of trays are set up in a sheet holding apparatus to hold a sheet. When holding in the sheets with the trays being controlled to move up and down, an initialization operation is carried out, on turning on electric power, or at the beginning of an image formation, to move the trays to an initialization position or a desirable position. The initialization operation is carried out in such a way that the trays, while in temporary up-and-down operation, are detected by a sensor, and then are moved up and down for a time period which matches the distance between a sensor and the initialization position or the desirable position after being detected by the sensor. However, the initialization operation suffers from shortcomings such as the operation is too long and too noisy.
Conventionally, a sheet holding apparatus is provided with, for example, 7 upper trays and 7 lower trays, in total 14 layers of trays. When the trays to be used are the 7 upper trays, in other words, when the number of copies set by an operator from the operation panel of the apparatus is 5 and a sort operation is chosen, the sheet holding apparatus determines it is sufficient to initialize only the 7 upper trays, and then choose the 7 upper trays to move up and down so as to identify the position by a sensor (e.g. refer to paragraphs [0214]-[0223] and FIG. 35 of JP-Tokukaihei-09-73251, hereafter referred as reference 1 in which the term of bin is used to refer to the term of tray which is used in the present application for holding an outputted sheet).
However, in the case of the reference 1, only the necessary trays (bins) corresponding to the number of copies of the sheets whereon an image is to be formed need to be initialized, so the initialization of the trays to be used is always moved to pass the same distance, the aforementioned shortcomings are not solved.